Overview of synthesizing hybrid nanomaterials with microfluidics

(Nanowerk Spotlight) Hybrid nanomaterials (nanohybrids) are composed of two or more components – at least one of which is nanoscale – exhibiting many distinct physicochemical properties and hold great promise for applications in optics, electronics, magnetics, new energy, environment protection, and biomedical engineering.

Different types of nanohybrids have been successfully synthesized via microfluidic or nanofluidic processes or hybrid microfluidic-batch processes. The synthesis of nanohybrids using microfluidic-based processes can fulfill many challenges present in conventional bottle batch methods.

A recent review article, by Junmei Wang and Yujun Song from the University of Science & Technology Beijing, in Small ("Microfluidic Synthesis of Nanohybrids") looks at the
features of the current types of microfluidic devices in the synthesis of different types of nanohybrids based on the classification of the four main kinds of materials: metal, nonmetal inorganic, polymer and composites.

In their review, the authors only illustrate some typical kinds of nanohybrids and the widely used nanohybrids – metal-organic frameworks (MOFs) – synthesized in microfluidic systems. Below is a summary of their article and their conclusions.

The regulation of the kinetic parameters in each stage of nanohybrid formation can be realized along microfluidic channels.

In the process, the integration of multiple microfluidic systems plays an important role in the optimization of the microstructures and properties of the nanohybrids.

Coupling of microfluidic systems with some special analysis devices aids online microstructure and performance observation or detection during the nanohybrid formation.

Moreover, the sequential synthesis and automation of the microfluidics during the entire procedure can provide a general low-cost and scale-out approach in the composition- component- and microstructure-controlled synthesis of nanohybrids with defined properties for advanced applications.

Despite the above mentioned advantages, some challenges still exist. For instance, it is still challenging to fabricate the microstructures of reactors and conducting the process optimization for the long-term synthesis of nanohybrids, especially magnetic nanohybrids without channel blockage and clogging in the current microfluidic reactors. In addition, it is still difficult to set up an ideal environment for the synthesis of nanohybrids (e.g., biomaterials) that need a desired biocompatible environment.

Even though multiphase microfluidic systems have been adopted to solve the above issues, some other problems may still exist. For example, it may be possible to synthesize nanohybrids without channel clogging using a multiphase microfluidic system (e.g., droplet processes) but it is difficult to add reagents subsequently after the droplets are formed.

Besides, it is still difficult to design a general microfluidic reactor system to synthesize nanohybrids with desired morphologies and microstructures since their complicated microstructures for different applications. Sometimes the process is usually tedious to synthesize nanohybrids with complicated constructions that may not be desired for practical applications.

So far, most microfluidic reactors only can be used at a relatively low temperature and low pressure. However, some synthesis processes need to be performed at very high temperatures and pressures (e.g., petroleum refinery or syngas reforming). A high growth temperature is also conducive to semiconductor nanohybrids with defect and/or doping control for good crystalline and excellent properties.

So far, it is easy to synthesize lots of nanohybrids with two components but still difficult to synthesize nanohybrids of multihierarchical microstructures or mesostructures.

It also is hard to realize that magnetic, optical properties, electrical properties integrated in a single nanohybrid.

Furthermore, the relationships between the surface and interface of different components and their interaction with their properties are still difficult to regulate even using multistep microfluidic processes.

Though online or in situ detection has been realized through integrated microfluidics, some operations are still inconvenient, such as in situ engineering surface and interface of nanohybrids. Therefore, many challenges have to be addressed to achieve a flexible and automated operation for composition and components control and their surface and interface engineering.

In addition, the integrated microfluidic devices are far from enough and limited currently. We believe that these integrated microfluidic systems will be more widely used in the sophisticated materials synthesis for advanced applications when overcoming these problems.

Concluding their reviw, the authors believe that these integrated microfluidic systems will be more widely used in the sophisticated materials synthesis for advanced applications when overcoming these problems.